1. Field of the Invention
The present invention relates to an improved process for extrusion of plasticized PVB comprising an adhesion control agent to a film for use in laminated safety glass.
2. Background Art
In the art of production and formulation polyvinyl acetals, it is widely accepted that acidic conditions are to be avoided in order to prevent a formed polyvinyl acetal from reacting back into polyvinyl alcohol and aldehyde. More specifically, in the production of polyvinyl butyral, the condensation reaction between butyric aldehyde and polyvinyl alcohol typically is catalyzed by mineral acids which during subsequent workup need to be carefully washed out and neutralized by alkaline compounds such as alkali metal hydroxides. Otherwise, strong acid easily promote cleavage of the butyral rings at elevated temperatures concomitant with yellowing. Under the conditions of extrusion, acid-containing PVB therefore easily turns yellow to brown and a certain amount of butyric aldehyde is cleaved from the polymer thus altering the properties of the PVB resin.
It is therefore common practice to employ PVB resin containing a certain amount of alkaline compounds, like alkali hydroxide. The degree of alkalinity in a PVB resin is usually measured and expressed as the “alkaline titer”, where positive titers indicate the presence of hydroxide and or other alkaline species such as for example carboxylate ions. Excess alkali metal salts can agglomerate and precipitate, causing “blushing.” EP 1022261 teaches extensive washing to remove these species, use of neutralizing agents such as epoxides rather than alkali hydroxide, or the addition of organic acids, phosphatic acids and amines to the PVB mixture as dispersion agent to keep the salts dispersed.
In the case of PVB film intended for use in laminated safety glass, sheet alkalinity can be further increased by addition of so called adhesion control agents (ACA). Commonly used for this purpose are carboxylates of either alkali or alkaline earth metals. It is understood that carboxylate ions act as weak bases and this contributes to sheet alkalinity. Potassium acetate or formate have been used in the past, but more recent PVB formulations mainly contain magnesium carboxylates, namely magnesium acetate and or magnesium salts of higher carboxylic acids such as 2-ethyl butyrate.
Regarding durability performance and robustness against defect formation for the finished laminates, use of magnesium ions typically results in superior properties compared to PVB film in which adhesion control is effected by potassium ions. More particularly, magnesium ions contribute less to edge whitening upon water ingress at the unprotected laminate edge, which easily occurs in humid environments. Also, the adhesive strength of such PVB film on the glass surface will be less dependent on the water concentration at the glass-PVB interface. But in general, a completely moisture independent adhesion cannot be achieved by addition of magnesium carboxylates. As a tendency, adhesion strength will be higher at lower moisture contents and vice versa. If a too dry PVB film is used for lamination of windscreens—e.g. after heating and stretching of the PVB which is commonly used to preform individual PVB blanks for lay-up—the resulting laminate adhesion may be too high and detrimental to the shock resistance of the laminate.
PVB film used for laminated glazing is produced by extrusion of a melt comprising PVB resin and plasticizer from a slit die, wherein the PVB resin can be pre-blended with plasticizer or simultaneously fed with liquid plasticizer into the extruder. Further additives such as heat stabilizers can be added to the mixture before extruding or can be supplied into the extruder with the plasticizer. For addition of ACA, it is common to feed the ACA as an aqueous solution in parallel to feeding the resin and the plasticizer.
However, using Mg-carboxylates as an ACA also causes problems in the extrusion process. It has been observed that after certain time of production, a whitish layer builds up inside the extruder and/or on the inner walls of the extrusion die and/or the die lip. This whitish layer makes it difficult to adjust and/or keep constant the thickness of the sheet as the melt exits the die.
Formation of the whitish layer triggers interruption of the production process due to the necessity to conduct some tedious cleaning to remove the layer. In other words, it is highly desirable to reduce and ideally completely avoid formation of such a whitish layer in order to increase up-time of production.
Chemical analysis of the whitish material has revealed that it contains Mg in very high concentration as high as 20% by mass. Although the exact chemical nature of the white material is not fully known, there is some indication that a form of Mg-hydroxide is its main constituent. Based on such observation it is speculated that the whitish residues are generated by hydrolysis of Mg-carboxylates into a form of Mg-hydroxide which in turn has reduced solubility in the PVB/plasticizer melt and will thus attach itself over time to the extruder and extrusion die walls. The source of hydroxide ions hydrolyzing the Mg-Carboxylate is thought to be residual alkali metal hydroxide added to the PVB resin to maintain a defined alkaline titer for preventing decomposition of the PVB polymer during extrusion.